CRM-13978 - Activity - Enable searching on activity result field.

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* CRM-13978: search on activity result field
  http://issues.civicrm.org/jira/browse/CRM-13978
CRM-13978 - Campaign - only build result drop down if needed

Avoid building if there are no options or if CiviCampaign is not
enabled.

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* CRM-13978: search on activity result field
  http://issues.civicrm.org/jira/browse/CRM-13978